1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to a flash memory, i.e., a nonvolatile semiconductor memory device having electrically and collectively erasable characteristics.
2. Description of the Related Art
Recently, in accordance with popularization of computers, word processors, and the like, a plurality of semiconductor memory devices, typically non-volatile semiconductor memory devices, e.g., flash memory, used in such information processors, have been developed and produced.
The flash memory, which is one kind of non-volatile semiconductor memory device, can be made programmable by the user. Further, the flash memory can be rewritten by electrically and collectively erasing the stored data, and then by programming. Therefore, the flash memories have attracted considerable attention as a replacement for magnetic storage devices because they are suitable for integration. Note, there is a necessity of improving redundant circuits, write circuits, and overerasing preventive measures for such a flash memory.